Everything was hard for me because I'm just super lazy and never cut down trees and stuff... haha. So whenever I had to place something HUGE, like Amir's Mansion or something, I had to spend like a whole extra day making space. So I guess the mansion is my answer!
Michelle's house was the first time I had difficulty placing a new building down. I had the town just the way I liked it and thought I had enough space for Michelle's house, but it wasn't. I had to spend some time moving stuff around and I liked having the town in one spot instead of two, so it took a while.